Since their introduction in the early 1970s, touch screens have afforded attractive alternatives to keyboards for certain computer applications. In many situations the keyboard and mouse are eliminated, because the touch screen provides the user with a much easier access to the computer. As a consequence, the market has grown to a substantial size, and a continued rapid growth is anticipated. However, current touch screens are difficult to produce, which creates a price barrier limiting growth into many new areas, such as education.
In this disclosure, a new concept is discussed that virtually eliminates design constraints and provides more freedom for the configuration of touch screens. Examples are given to illustrate this new freedom in design parameters. These design concepts provide a basis for producing touch screens at a much lower cost, without sacrificing quality. Furthermore, the creation of new designs for special sensor size, shape, or electrical characteristics is greatly simplified with the concept described herein and reduces research and development costs.
A substantial portion of the touch screens produced today are based on the measurement of electrical potentials on substrates that are made of a transparent medium such as glass, coated with an electrically conductive material. Uniform electrical fields must be maintained on the substrate, and these are applied sequentially in the x- and y-directions.
In other words, equally spaced equipotential lines are generated orthogonally in a timed sequence. A voltage (or equivalently, a current related to the local potential of the touch point) measured when the field is in the x-direction is directly proportional to the distance along the x coordinate and is independent of the y coordinate. Conversely, a voltage measured when the field is in the y-direction is directly proportional to the distance along the y coordinate and is independent of the value of x.
According to present designs, resistive touch screens are often mounted on LCD or CRT displays, but perhaps most commonly on CRTs used as computer monitors to use as data input devices. As shown in FIG. 6 a typical monitor 10 will comprise a back case 11 into which is set the CRT. A glass panel 12 with a uniform resistive coating 15 (shown in FIG. 7) such as ITO (indium tin oxide) is placed over the face 14 CRT 13. A polyester cover sheet is tightly suspended over the top of the glass panel, preferably separated from it by small transparent insulating dots 16 as described in Hurst, U.S. Pat. No. 3,911,215 which is incorporated herein by reference. The cover sheet 17 has a conductive coating on the inside and a hard durable coating 18 on the outer side. A more detailed view of the layers of the touch screen is shown in FIG. 7, with a bezel 19.
A simple computer or controller 20 (shown in FIG. 8) is used to alternate a voltage across the resistive surface of the glass in the X and Y directions. When a touch on the cover sheet causes the inner conductive coating to make electrical contact with the resistive coating on the glass, an electrical circuit connected to the controller digitizes these voltages or equipotentials and transmits them to the associated main computer 21 for processing. As shown in FIGS. 8A and 8B, the controller 20 may be mounted internal to the monitor 10 or in a slot within the associated main computer 21.
In practice, the implementation of these concepts, as disclosed in the Patent of Hurst (U.S. Pat. No. 3,798,370, March, 1974) leads to the production of touch screens of excellent quality. Yet production costs are high, because of three factors:
1) The substrate must have very uniform conductivity. Conductive materials are applied to a substrate (usually glass) in elaborate coating chambers. When a large substrate is being prepared, the chamber must be still larger, and even then, several sources must be used to cover the substrate uniformly. Some of these coated substrates do not meet specifications and have to be rejected.
2) A resistor divider network must be added to maintain straight equipotentials by eliminating edge effects associated with the field switching matrix. This has independent quality demands that further add to production costs and increase rejection rates.
3) Finally, rigorous testing must be done on the substrate itself and on each completed screen. These statistical quality-control tests are expensive and are directly associated with the problem of maintaining accurate equipotentials.
Currently, design changes requires considerable retooling. However, retooling costs and delays are considerably reduced by using the new concepts in the present invention. These concepts, to be explained, will reduce all of the cost factors and, at the same time, provide much more flexibility in the design of sensors of the required shape, size, and electrical specifications.
It is therefore a purpose of the invention to provide improved touch screen production by enhancing screen yield through an inherent tolerance for individual and lot variances. It is a further object of the invention to permit simplified manufacture requirements for touch screens including less-demanding conductive-coating application; fewer and much simpler electrodesxe2x80x94only four, for example, or even a simple resistance framing design; with no divider resistors required. It is yet another purpose of the invention to provide compatibility with current manufacture of analog-to-digital electronics, and calibration/testing procedures. It is yet another object of the invention to permit manufacture at low additional cost, more than offset by savings in screen manufacture. It is another purpose of the invention to permit liberated design of touch screens with changes readily implemented to accommodate new screen configurations.